The present invention represents an improvement and further extension of the basic dual flow principle for solar radiation collection systems described in U.S. Pat. No. 4,054,124, previously issued to patentee. The referenced patent describes how high performance can be achieved, with a very low time constant, by employing the dual flow principal in conjunction with other factors. Among these factors, the height of the mass flow channel under the collector, with which the bypass flow through apertures in the panel face is to be combined, is specified as having a preferred range of 0.5 to 2.0 mm, the f number (or proportion of the bypass flow to full flow) is discussed in terms of the range of 0.25 to 0.75, and examples of bypass apertures of 0.5 are given. While confirming the validity of the concept and the value of the implementations taught in U.S. Pat. No. 4,054,124, the present application also discloses further improvements pertaining to such structures, particularly as applied to compact modular units that are particularly low in cost, readily fabricated, and ideally suited to more demanding environments, such as the weak insolation conditions existing at northern latitudes. Details of structures incorporating the concepts disclosed herein may be appreciated by reference to a concurrently filed application entitled "Modular Solar Energy Collector Systems" Ser. No. 856,256, filed Dec. 1, 1977 by the inventor hereof and assigned to the same assignee.